Do you maybe mean ‘caterpillar drive’ or ‘magnetohydrodynamic drive’?
Caterpillar drive may be silent, but it isn’t fast. It could never get close to 50 kts.
I had thought about the issue of the sub being blind at that speed. But it is a drone and would be on a predetermined course. Cruising at depth on the predetermined route its only hazards would be whales and other subs.
In a war in which this weapon would be used it may be considered an acceptable risk.
And I am guessing that its speed is for purely defensive purposes; escaping a sub killer. Short sprints not long marathons.
Until they get a sonar that is silent, any sub at high speed just means you have to lead them a little more with a offensive weapons shot.. Not faster that a sub rock...